The present invention relates to the field of electrical circuitry, and more particularly relates to the fabrication of individual panels with multiple layers.
There continues to be an ever increasing need for multi-layer electronic assemblies which provide high density, complex interconnections for electrical components. Methods for forming multilayer electronic assemblies are taught in Dux et al., U.S. Pat. No. 5,224,265 and Ehrenberg et al. U.S. Pat. No. 5,232,548. The ""265 patent discloses a high-density, multi-layer thin film structure and process for making a multi-layer structure wherein multiple individually testable sub-units are fabricated in parallel, pre-tested for operational performance, and joined together to form a three dimensional wiring matrix.
Commonly assigned U.S. Pat. No. 5,590,460 to DiStefano et al., the disclosure of which is incorporated by reference herein, teaches a process for making a multi-layered assembly including providing a sheet of a dielectric material having a top and a bottom surface. Layers of a temporary layer susceptible to etching are coated on the top and bottom surfaces of the dielectric sheet, respectively. Photoresist layers are then applied over the top and bottom temporary layers and are photographically exposed to develop openings which are perpendicular to the external surfaces of the assembly. The assembly is then expressed to an etchant to form aligned apertures in the top and bottom temporary layers. After formation of the holes, an electrically conductive structural material such as a metal is deposited within each hole and within the apertures in the top and bottom temporary layers. A structural material is then applied as a continuous layer covering the peripheral surfaces of the holes in the sheet and also covering the peripheral walls of the apertures in the temporary layers.
Despite these and other efforts in the art, there are needs for still further improvement.
According to one embodiment of the invention, a method of making a multi-layer circuit assembly includes providing a core structure having an inner dielectric element with first and second metal layers on opposite surfaces thereof. The dielectric element, such as a polyimide material, is preferably between about 25 to 50 microns thick. The first and second metal layers may comprise a highly conductive material such as copper and each metal layer is generally about are 1 to 18 thick. In one embodiment, the first metal layer on top of the dielectric element could serve as a ground plane and the second metal layer on the bottom surface of the dielectric element could serve as a power plane. The first and second metal layers will also act as a backbone for the final assembly to provide rigidity thereto. The specific composition of the first and second metal layers may vary depending upon the specific application for which the final assembly will be used, which in turn will determine the thermal coefficient of expansion and the ultimate dimensional stability of the assembly.
In the next stage of the process, one or more through vias are formed through the metal layers and the inner dielectric element. The through vias may be created by a variety of well know techniques such as punching, laser during or plasma etching. The through vias should extend entirely through the first metal layer, the dielectric element and the second metal layer. The exact location of the through vias may vary depending upon the specific requirements of the final assembly. In one preferred embodiment, the through vias may be formed by first etching the metal layers to form aligned holes therein, aligning a laser with the aligned holes and laser drilling through the dielectric element. In another embodiment, a YAG laser may be used. In a first drilling step, the first and second metal layers are drilled while the YAG laser is on high power. In a second drilling step, the dielectric element is drilled while the YAG laser is on low power. As will be discussed in more detail below, although laser drilling may be slightly more costly than plasma etching, laser drilling provides for better alignment of the holes in the metal layers with the holes in the dielectric element. In addition, the side walls formed using a laser are more uniform than those formed with etching techniques.
After the through vias have been formed, the first and second metal layers and the through vias are coated with a dielectric material, such as a polyimide or an epoxy, to form a coated structure having first and second outer dielectric layers. The dielectric material is applied using well known techniques such as dipping, spin coating and plating techniques (e.g. preferably electrophoretic plating). During the coating process, all of the exposed surfaces of the metal layers, including the side walls of the through vias, are covered by the dielectric material. The thickness of the dielectric material coating should be uniform. For example, in certain preferred embodiments, after the coating step, the outer dielectric layers have a uniform thickness of approximately 25 to 75 microns. The exact thickness of the dielectric material is controlled so that the through vias remain open after the coating process. For example, in certain embodiments, the diameter of the through vias is approximately 175 to 200 microns before the coating process and approximately 25 to 150 microns after the coating process.
In the next stage of the process, outer metal layers comprising highly conductive materials such as copper are provided over the first and second outer dielectric layers, respectively. The step of providing the outer metal layers over the first and second outer dielectric layers may include the steps of seeding the exterior surface of the outer dielectric layers, such as by exposure to a liquid seeding process or by sputter deposition. For example, an adhesion promoting layer of a conductive base, such as a graphite seed, may be applied. The seed layer contacts the first and second outer dielectric layers. The outer metal layers are then electroplated over the seed layer as by connecting an electroplating potential source to the first and the second metal layers and immersing the assembly in an electroplating bath with a counter electrode. The outer metal layers may also be provided using electroless plating or sputtering techniques. The outer metal layers substantially conform to the shape of the first and second outer dielectric layers. Each outer metal layer preferably has a thickness of approximately 1 to 18 microns and most preferably a thickness of approximately 1 to 6 microns; however, the exact thickness of the outer metal layers will depend upon the particular application for which the assembly will be used. Finally, the coated through vias are metallized to form metallic via liners which are connected to the outer metal layers and which are insulated from the first and second metal layers.
In other preferred embodiments, the adhesion between the outer metal layers and the first and second outer dielectric layers may be enhanced by providing a tie coat including nickel and chrome. In these embodiments, the nickellchrome tie coat is provided directly over the outer dielectric layers and then a seed layer, such as copper, is provided over the tie coat. Finally another layer of copper, which is thicker than the seed layer, is provided over the seed layer.
In the next stage of the process signal lines are formed by selectively patterning the outer metal layers to form first signal lines overlying, the first metal layer and second signal lines overlying the second metal layer. The selectively patterning step includes the step of selectively removing portions of the outer metal layers such as by selectively etching the outer metal layers. In one embodiment, a photo-resist layer is provided over the outer metal layers and the layers are then subjected to an etching process to pattern the signal lines therein. In other embodiments the selectively patterning step may include the step of selectively depositing the outer metal layers. The first signal lines are substantially parallel to one another and substantially perpendicular to the second signal lines. In turn, the second signal lines are substantially parallel to one another. After the signal lines have been formed, the assembly may be used to electrically connect circuit elements on the same side of the assembly or on opposite sides thereof. The above-described methods provide economical processes of making a multi-layer circuit assembly. The same techniques may be used with a different number of layers to provide a circuit assembly having numerous layers whereby several of the structures described above can be laminated together. For example, the lamination techniques disclosed in U.S. Pat. No. 5,282,312, the disclosure of which is incorporated herein by reference, may be employed.
In other embodiments, blind via openings may be formed through the outer dielectric layers to expose one or more regions of the first and second metal layers. The blind vias may be formed by placing a mask over the first and second outer dielectric layers and scanning a laser over the mask so that laser light passes through apertures in the mask to cut through the dielectric material. During the laser drilling step, substantially all of the dielectric material is removed from the blind vias; however, a slight residue of dielectric material may remain at the closed ends of one or more of the blind vias adjacent the exposed regions of the first and second metal layers. The residue in the blind vias, which typically has a thickness of less than one micron, may be removed by subjecting the assembly to a plasma etching process. Although it is preferable to remove the residue using a plasma etching process so that all of the blind vias may be simultaneously treated, it is also possible to use a YAG laser to remove the residue. However, the use of a laser to remove the residue may prove time consuming because typically the laser must be aligned with each of the blind via openings. After the blind vias have been formed, one or more regions of the first and second metal layers are exposed and accessible through the blind vias. The blind vias are then metallized so that at least some of the signal lines are connected to the first and second metal layers. The steps of metallizing the through vias and metallizing the blind vias may be performed simultaneously.
In another embodiment of a method of making a multi-layer circuit assembly, the method includes providing an inner dielectric element and first and second metal layers having openings therein on opposite surfaces of the inner dielectric element, whereby each opening in the first metal layer is in substantial alignment with one of the openings in the second metal layer. For example, first and second metal layers having a plurality of apertures therein may be provided and the metal layers may be laminated to opposite surfaces of a dielectric element so that the openings in the first and second metal layers are in substantial alignment. Lamination techniques are disclosed in commonly assigned U.S. Pat. No. 5,583,321, the disclosure of which is incorporated herein by reference The inner dielectric element and the first and second metal layers are then coated with a dielectric material, using the coating processes described above, to thereby form a coated structure having first and second other dielectric layers overlying the first and second metal layers respectively. Next, through vias are formed through the coated structure, whereby each through via is in substantial alignment with the aligned openings in the first and second metal layers. For example, a laser may be aligned with the aligned openings in the metal layers for simultaneously drilling through the outer metal layers and the dielectric element to provide one or more through vias. During the drilling step, it is critical that the diameter of the laser cut be controlled so that the first and second metal layers remain covered by the dielectric material and are not exposed at the side walls of the through vias. In alternative embodiments, the through vias may be formed using the etching or punching techniques described above. After the through vias have been formed, a final multi-layer assembly may be fabricated using the processes described above.
In another embodiment of the present invention, the first and second outer metal layers may be provided over the outer dielectric layers before the blind vias are formed in the outer dielectric layers. The outer metal layers according to this embodiment may be laminated to the outer dielectric layers or may be applied using a plating technique or sputtering technique. The blind vias may then be formed through the outer metal layers and the outer dielectric layers using the various techniques described above in order to expose one or more regions of the first and second metal layers. Next, the blind vias may be metallized so that at least some of the signal lines are connected to the first and second metal layers. The metal in the blind vias may be provided using electroless plating and sputtering techniques.
In other embodiments, additional signal lines may be formed in at least one of the first and second metal layers before the coating step.
For example, signal paths for carrying critical signals may be etched into the first and second metal layers, and the signal paths insulated during the coating with a dielectric material step.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.